plainLength method
Plaintext length of a vault file whose total size is fileLength.
Implementation
int plainLength(int fileLength) {
final int body = fileLength - length;
if (body <= 0) return 0;
final int full = body ~/ sealedChunkSize;
final int rest = body % sealedChunkSize;
return full * chunkSize + (rest == 0 ? 0 : rest - _tagLength);
}